Crude oil and hydrocarbons, fractional distillation, complete and incomplete combustion, the pollutants from burning fuels, and their environmental effects.

What this dot point is asking

Edexcel wants you to describe crude oil as a mixture of hydrocarbons, explain how it is separated by fractional distillation, write the products of complete and incomplete combustion, and describe the pollutants from burning fuels and their effects.

Crude oil and hydrocarbons

Fractional distillation

Crude oil is separated into useful fractions by fractional distillation, which works because the hydrocarbons have different boiling points (larger molecules boil at higher temperatures). The oil is heated and the vapours rise up a column that is hot at the bottom and cool at the top; each fraction condenses where the temperature matches its boiling point. Fractions include refinery gases, petrol, kerosene, diesel and bitumen.

Combustion

Fuels burn (combust) in oxygen, releasing energy:

• Complete combustion (plenty of oxygen): hydrocarbon + oxygen $\rightarrow$ carbon dioxide + water. This releases the most energy.
• Incomplete combustion (limited oxygen): also produces carbon monoxide (toxic) and carbon (soot), and releases less energy.

Pollutants and their effects

Burning fuels releases several pollutants:

• Carbon dioxide ($CO_2$): a greenhouse gas that contributes to global warming.
• Carbon monoxide ($CO$): a toxic gas (from incomplete combustion) that reduces the oxygen the blood can carry.
• Soot (carbon): causes breathing problems and dirties buildings.
• Sulfur dioxide ($SO_2$) and nitrogen oxides: dissolve in rain to form acid rain, which damages plants, buildings and aquatic life. Nitrogen oxides form in hot engines.

Sulfur dioxide comes from sulfur impurities in the fuel, so removing sulfur before burning reduces it. Nitrogen oxides form when the high temperature inside an engine makes the nitrogen and oxygen in the air react together, which is why they are produced even though the fuel itself contains no nitrogen. Because crude oil is a finite resource that took millions of years to form, and because burning it releases these pollutants, there is growing interest in alternative and renewable energy sources. Understanding where each pollutant comes from is the key to explaining how it can be reduced, which is a common higher-mark exam question.

Try this

Q1. Define a hydrocarbon. [1 mark]

• Cue. A compound made of hydrogen and carbon only.

Q2. Name the toxic gas produced by incomplete combustion. [1 mark]

• Cue. Carbon monoxide.